Field of the Invention
The present invention relates to a photoacoustic apparatus that receives a photoacoustic wave generated when an object is irradiated with light and obtains object information, a signal processing method, and a storage medium storing a program.
Description of the Related Art
When an object such as a living matter is irradiated with light, the light propagated and diffused in the object is absorbed, and a pressure is generated, so that a photoacoustic wave is generated. In recent years, an apparatus using this photoacoustic effect has been actively researched and developed. In particular, photo-acoustic imaging (PAI) has attracted attention in which the generated photoacoustic waves are received from various directions, and analysis processing of the received photoacoustic waves is performed, so that a spatial distribution of object information in the object can be obtained.
Among the object information obtained by the PAI, an absorption coefficient distribution is much anticipated. The optical absorption is mainly derived from hemoglobins (oxyhemoglobin, deoxyhemoglobin) in blood vessels, and if the absorption coefficient distribution is measured while a wavelength of the light is changed, it is possible to obtain concentration distributions of the respective hemoglobins. In addition, it is possible to obtain a ratio of oxygenated hemoglobin in total hemoglobin, that is, an oxygen saturation distribution on the basis of the concentration distributions of the respective hemoglobins. To accurately obtain the above-described object information, the absorption coefficient distributions in the respective wavelengths are accurately obtained.
Hereinafter, a method of obtaining the absorption coefficient distribution will be described. To facilitate understanding, first, phenomena that actually occur will be explained in order, and next, a method of obtaining the absorption coefficient distribution will be described so as to trace back the phenomena. As mention above, the photoacoustic effect is a phenomenon where an object is irradiated with light, the light is propagated and diffused, the light is absorbed, a pressure is generated, and a photoacoustic wave is generated. It is known that propagation and diffusion of the light at visible to near-infrared wavelengths in the living matter except for an extremely shallow part can be satisfactorily approximated by an optical diffusion equation, and an optical diffusion equation in a stationary state can be written as in Expression (1).−∇·{D({right arrow over (r)})∇ϕ({right arrow over (r)})}+μa({right arrow over (r)})ϕ({right arrow over (r)})=s({right arrow over (r)})  (1)
Where D denotes a diffusion coefficient distribution, φ denotes an optical fluence distribution in the object, μa denotes an absorption coefficient distribution in the object (real absorption coefficient distribution), s denotes a source term of the light, and r denotes a position. The pressure, the absorption coefficient, and the optical fluence have a relationship as represented in Expression (2).p0({right arrow over (r)})=Γ({right arrow over (r)})μa({right arrow over (r)})ϕ({right arrow over (r)})  (2)
Where p0 denotes a real initial acoustic pressure distribution, and Γ denotes a Gruneisen coefficient distribution. This generated photoacoustic wave at the real initial acoustic pressure p0 propagates through the object to be measured by an acoustic wave receiver.
The absorption coefficient distribution is obtained so as to trace back this. First, a time variation of the acoustic pressure of the photoacoustic wave propagating through the object is measured, and the acoustic pressure distribution generated in the object is obtained from the measurement result by a method as described in C. Li, L. V. Wang, “Photoacoustic tomography and sensing in biomedicine”, Phys. Med. Biol. 54 (2009) R59, which will be referred to as Non-patent document 1. This distribution will be referred to as initial acoustic pressure distribution δp0.
If the initial acoustic pressure distribution δp0 is considered as the real initial acoustic pressure distribution p0, an absorption coefficient distribution μa can be obtained by dividing the initial acoustic pressure distribution δp0 by the optical fluence distribution φ and the Gruneisen coefficient distribution Γ on the basis of Expression (2). However, as may be understood from this procedure, to obtain the absorption coefficient distribution μa from the initial acoustic pressure distribution δp0, the optical fluence distribution φ is to be obtained. In view of the above, the optical fluence distribution φ is obtained by using Expression (1). For example, μaφ in Expression (1) is transformed into p0/Γ on the basis of Expression (2) to form Expression (3).−∇·{D({right arrow over (r)})∇ϕ({right arrow over (r)})}+p0({right arrow over (r)})/Γ({right arrow over (r)})=s({right arrow over (r)})  (3)
Therefore, if the initial acoustic pressure distribution δp0 is considered as the real initial acoustic pressure distribution p0, that is, considered as Expression (4), Expression (4) can be solved with respect to the optical fluence distribution φ to be obtained.−∇·{D({right arrow over (r)})∇ϕ({right arrow over (r)})}+δp0({right arrow over (r)})/Γ({right arrow over (r)})=s({right arrow over (r)})  (4)
In this manner, by solving the optical propagation equation including the initial acoustic pressure distribution δp0, it is possible to obtain the optical fluence distribution φ.